Get child node of SKReferenceNode in SpriteKit SWIFT

I create a scene Case.sks (using Level Editor), inside one SKSpriteNode (name : square), and one SKLabel (name : label).
In my main scene, GameScene.sks, I use a SKReferenceNode with "Case" for reference.

I need to access to the "square" sprite from my main scene.

My first idea was to call directly the child node:

let firstSquare = childNode(withName: "square") as! SKSpriteNode

But I got :

Fatal error: unexpectedly found nil while unwrapping an Optional value